1. Field of the Invention
The present invention relates to a device for controlling a stepping motor used, for example, for conveying a sheet of recording paper in a copy machine. More specifically, the present invention relates to a control device for a stepping motor enabling reduction of power loss and reduction of ripple current of a stepping motor driving IC at the time of acceleration and deceleration without necessitating the conventional current switching circuit, as well as to a conveyer device provided with the control device.
2. Description of the Background Art
As one type of image processing apparatuses as electronic equipment, an MFP (MultiFunction Peripheral) has been known. An MFP has a plurality of functions, including copy, print, facsimile (hereinafter also denoted as FAX) and scanner functions. The MFP allows a user to readily copy, send FAX or print. In MFPs and the like, a stepping motor is used as a motor for driving rollers for conveying recording paper and for automatically reading documents.
Generally, high torque is required at the time of acceleration/deceleration of a stepping motor (when the stepping motor is accelerated or decelerated). The stepping motor is controlled such that a motor control current is increased at the time of acceleration/deceleration to increase motor output, and the motor control current is decreased at a steady state (while the motor is rotating at a constant speed), as the torque becomes lower, for example, than at the time of activation. For this purpose, a conventional stepping motor control circuit is provided with a current switching circuit, and current value of the stepping motor is switched in accordance with the state of the stepping motor (whether it is accelerated/decelerated or in a steady state).
Further, a method of control that decreases lost heat of an element by lowering PWM frequency while a high current is caused to flow during acceleration/deceleration has been proposed. For example, Japanese Patent Laying-Open No. 2004-7894 (hereinafter referred to as '894 Reference) discloses a control device that lowers PWM frequency at the time of activation, as a solution to a problem that, without any function of switching the PWM frequency, if the PWM frequency is set high to increase accuracy of rotation, a high current is caused to flow at the time of activating a motor, and power loss undesirably increases when a driving transistor switches.
As described above, conventional control is such that the current value is decreased when the stepping motor is accelerated/decelerated. Such control requires a current switching circuit for switching the current value. Further, even when the value of effective current is the same as at the steady state, peak current increases during acceleration and, therefore, a power source that can supply the high peak current becomes necessary. In addition, there is a problem that ripples of driving current become larger during acceleration than in the steady state. These problems cannot be solved by the technique disclosed in '894 Reference.